Substrates for semiconductor devices

ABSTRACT

A method of fabricating a composite semiconductor component comprising: (i) providing a bowed substrate comprising a wafer of synthetic diamond material having a thickness t d , the bowed substrate being bowed by an amount B and comprising a convex face and a concave face; (ii) growing a layer of compound semiconductor material on the convex face of the bowed substrate via a chemical vapour deposition technique at a growth temperature T to form a bowed composite semiconductor component comprising the layer of compound semiconductor material of thickness t sc  on the convex face of the bowed substrate, the compound semiconductor material having a higher average thermal expansion coefficient than the synthetic diamond material between the growth temperature T and room temperature providing a thermal expansion mismatch ΔT ec ; and (iii) cooling the bowed composite semiconductor component, wherein the layer of compound semiconductor material contracts more than the wafer of synthetic diamond material during cooling due to the thermal expansion mismatch ΔT ec , wherein B, t d , t sc , and ΔT ec  are selected such that the layer of compound semiconductor material contracts on cooling by an amount which off-sets bowing in the bowed substrate thus pulling the bowed composite semiconductor component into a flat configuration, the layer of compound semiconductor material having a tensile stress after cooling of less than 500 MPa.

FIELD OF INVENTION

The present invention relates to the manufacture of substrates forsemiconductor devices.

BACKGROUND OF INVENTION

Optoelectronic, high power, and high frequency devices are increasinglybeing fabricated using wide band gap compound semiconductor materialssuch as gallium nitride, aluminium nitride, and silicon carbide. Suchsemiconductor materials are frequently grown heteroepitaxially in thinfilm form onto a suitable substrate which provides a template forcrystal growth. Typical substrates include sapphire, silicon carbide,and silicon. For semiconductor devices such as microwave amplifiercircuits, the substrate should be electrically insulating for the deviceto function.

A well known problem in semiconductor devices is that of heatdissipation. High temperatures often limit the performance and/orlifetime of such devices. This is a particular problem in semiconductordevices which operate at high power and/or high frequency such asmicrowave amplifiers, power switches and optoelectronic devices. It istherefore desirable to be able to spread any heat generated by componentdevices to reduce temperatures and thus improve device performance,increase device lifetime, and/or increase power density. Accordingly, itis desirable to utilize a substrate material with a high thermalconductivity to spread the heat generated by a device, lowering thepower density and facilitating dissipation via a heat sink thusimproving device performance, increasing lifetime, and/or enabling anincrease in power density.

Diamond has unique properties as a heat spreading material, combiningthe highest room temperature thermal conductivity of any material, withhigh electrical resistivity and low dielectric loss when in an intrinsicundoped form. Thus diamond is utilized as a heat spreading substrate forsemiconductor components in a number of high power density applications.The advent of large area polycrystalline diamond produced by a chemicalvapour deposition (CVD) technique has expanded the applications fordiamond heat spreaders via an increase in area and a reduction in cost.The majority of favourable thermal, dielectric and insulating propertiesof diamond are not dependent on the single crystal structure ofnaturally occurring or synthetic single crystal diamond material.Accordingly, polycrystalline CVD diamond wafers have been developed andare commercially available in sizes that enable them to be directlyintegrated with the fabrication processes of wide band gapsemiconductors as a substrate material.

In light of the above, it is evident that for thin film compoundsemiconductor materials, an ability to integrate diamond as a carriersubstrate could greatly improve thermal performance. For high powerdevices, the challenge is to position an active region of a device in asclose proximity as possible to the heat spreading diamond substrate,since any intermediate carrier substrate material such as sapphire,silicon, or silicon carbide acts as a thermal barrier.

Compound semiconductor materials can be grown directly on apolycrystalline diamond substrate using, for example, metal organicchemical vapour deposition (MOCVD) technique. Alternatively, a thinlayer of monocrystalline material such as silicon, silicon carbide, or anitride can be disposed on a polycrystalline diamond substrate andcompound semiconductor material epitaxially grown on the thin layer ofmonocrystalline material.

U.S. Pat. No. 7,595,507 and US 2010/0001293 disclose methods of formingsemiconductor device substrates which comprise growing diamond over amonocrystalline silicon carbide layer. US 2009/0272984 also discloses amethod of forming a semiconductor device substrate comprising diamondand silicon carbide. Such composite diamond-silicon carbide substratescan be used to form semiconductor devices. When forming such devices,the mono-crystalline silicon carbide layer can be used to grow amonocrystalline semiconductor layer thereover.

US 2006/0113545 discloses substrate structures comprisingsilicon-diamond-silicon multilayer structures. One problem with usingsilicon is that it offers relatively poor thermal and resistivityproperties compared to silicon carbide. As such, it is desirable to makethe silicon layer very thin such that the polycrystalline diamond willbe disposed close to the active semiconductor components to effectivelydissipate heat generated during operation.

Various prior art documents disclose methods of growing polycrystallinediamond on a substrate, reducing the thickness of the substrate to forma thin single crystal layer disposed on the polycrystalline diamond, andthen growing active semiconductor layers on the thin single crystallayer. Examples of such prior art documents are briefly discussed below.

WO 2005/122284 and WO 2006/100559 disclose growth of polycrystallinediamond material on a silicon wafer followed by thinning of the siliconwafer by grinding or lapping to achieve a thin layer of silicon disposedon the polycrystalline diamond material.

EPO442304 discloses growth of polycrystalline diamond material on asilicon wafer. It is described that a thin layer of single crystalsilicon carbide forms at an interface between the silicon and diamondmaterial during growth. The document suggests that the silicon wafer canbe removed to leave the thin layer of silicon carbide adhered to thepolycrystalline diamond material and this thin layer of silicon carbidecan be used as a growth surface for fabrication of semiconductor layers.The present inventors consider that this is incorrect as they have foundthat while a thin layer of silicon carbide does form at the interfacebetween the silicon and diamond materials, this layer is amorphous andfound the latter not suitable for fabricating single crystalsemiconductor materials thereon by epitaxial growth.

WO2005/074013 and US2009/0272984 disclose ion implanting a buried SiO₂layer into a silicon carbide wafer, growing polycrystalline diamondmaterial on the wafer, and then removing the bulk of the silicon carbidewafer by using the implanted SiO₂ layer as a release layer.

U.S. Pat. No. 7,595,507 discloses ion implanting a buried SiO₂ layer of100 nm to 200 nm thickness into a silicon wafer, growing polycrystallinediamond material on the wafer, and then removing the bulk of the siliconwafer using a wet etch with the thin buried SiO₂ layer acting as an etchstop to achieve a thin layer of silicon disposed on the polycrystallinediamond material.

U.S. Pat. No. 7,695,564 discloses a similar method which comprisesion-implantation of oxygen into a silicon wafer to form a wafercomprising a bulk silicon wafer layer of an unspecified thickness, aburied oxide layer having a thickness of approximately 100-200 nm, and asilicon overlay structure having a thickness of 50-500 nm. Apolycrystalline diamond film of approximately 200 to 1500 micrometers isgrown on the silicon overlayer. The bulk silicon wafer layer and theburied oxide layer are then removed to leave a composite structurecomprising the polycrystalline diamond film and the silicon overlayer. Anumber of different methods are described for removing the silicon toform a diamond substrate with a thin silicon overlayer including: (1)selectively dissolving the buried oxide layer; (2) wet etching the bulksilicon layer followed by wet etching of the buried oxide layer; (3)lapping and polishing the bulk silicon layer followed by wet etching; or(4) lapping and polishing the bulk silicon layer followed by dryetching. The finished silicon-on-diamond substrate consists of anapproximately 50 to 200 nm thick monocrystalline silicon film which isepitaxially fused to an approximately 300 to 1500 micrometer thickpolycrystalline diamond substrate.

While it would seem to be simple in principle to apply any of theaforementioned techniques to thin down a substrate wafer afterpolycrystalline diamond growth thereon to achieve a polycrystallinediamond wafer with a thin layer of the substrate wafer adhered thereto,in practice a problem exists with known prior art techniques. Namely, asthe substrate wafer is thinned down cracking occurs in the thin layersuch that although a thin layer is achieved is not of high quality. Thisis problematic because the quality of the thin layer will affect thequality of the semiconductor layers epitaxially grown thereover to forman electronic device and this detrimentally affects device performance.In particular, the present inventors have found that as the singlecrystal layer adhered to the polycrystalline diamond material is thinnedto a depth of less than 100 μm cracks begin to form in the thin layer ofsingle crystal material. While it was initially thought that suchcracking may be a result of mechanical damage caused by grinding,lapping, and polishing techniques, it has been found that this sameproblem occurs when using etching techniques such as those describedabove.

Another problem with prior art methods is that compound semiconductormaterials grown on substrates comprising diamond wafers tend to yield alayer of compound semiconductor material which exhibits a high degree oftensile stress. The present inventors have identified that this iscaused by a thermal expansion coefficient mismatch between the diamondmaterial and the compound semiconductor material. Specifically, compoundsemiconductor materials such as GaN have a higher thermal expansioncoefficient than diamond material between room temperature and thegrowth temperature at which the compound semiconductor material isdeposited. Compound semiconductors such as GaN can be deposited by anumber of techniques including chemical vapour deposition (CVD),molecular beam epitaxy, and sputtering and growth temperatures can be inexcess of 1000° C. On cooling after CVD growth of the compoundsemiconductor material on a suitable substrate, the compoundsemiconductor layer and the substrate contract. If the substratecomprises a diamond wafer, the diamond material does not contract asmuch as the compound semiconductor layer would natural do as diamondmaterial has a much lower thermal expansion coefficient than typicalcompound semiconductors. The diamond substrate wafer therefore generatestensile stress within the compound semiconductor layer during coolingand this can affect the performance of the layer in electronicapplications. Furthermore, the tensile stress in the compoundsemiconductor layer can result in cracking of the layer during coolingor subsequent handling.

It is an aim of certain embodiments of the present invention to solveone or more of the aforementioned problems and provide a method offabricating a high quality, crack free, low tensile stress layer ofcompound semiconductor material on a substrate comprising a wafer ofpolycrystalline diamond material. Advantageously, the layer of compoundsemiconductor material is disposed directly on the wafer ofpolycrystalline diamond material or positioned close thereto with one ormore very thin intermediate bonding layers disposed between the compoundsemiconductor and diamond material. The one or more intermediate bondinglayers may be formed from single crystal materials such as silicon,silicon carbide, or nitrides suitable for epitaxial growth of compoundsemiconductors.

SUMMARY OF INVENTION

According to a first aspect of the present invention there is provided amethod of fabricating a composite semiconductor component comprising:

-   -   (i) providing a bowed substrate comprising a wafer of synthetic        diamond material having a thickness t_(d), the bowed substrate        being bowed by an amount B and comprising a convex face and a        concave face;    -   (ii) growing a layer of compound semiconductor material on the        convex face of the bowed substrate via a chemical vapour        deposition technique at a growth temperature T to form a bowed        composite semiconductor component comprising the layer of        compound semiconductor material of thickness t_(sc) on the        convex face of the bowed substrate, the compound semiconductor        material having a higher average thermal expansion coefficient        than the synthetic diamond material between the growth        temperature T and room temperature providing a thermal expansion        mismatch ΔT_(ec); and    -   (iii) cooling the bowed composite semiconductor component,        wherein the layer of compound semiconductor material contracts        more than the wafer of synthetic diamond material during cooling        due to the thermal expansion mismatch ΔT_(ec),        wherein B, t_(d), t_(sc), and ΔT_(ec) are selected such that the        layer of compound semiconductor material contracts on cooling by        an amount which off-sets bowing in the bowed substrate thus        pulling the bowed composite semiconductor component into a flat        configuration, the layer of compound semiconductor material        having a tensile stress after cooling of less than 500 MPa.

Accordingly to a second aspect of the present invention there isprovided a composite semiconductor component comprising:

-   -   a substrate comprising a wafer of synthetic diamond material;        and    -   a layer of compound semiconductor material on the substrate,    -   wherein the layer of compound semiconductor material has a        tensile stress of less than 500 MPa at room temperature (25°        C.).

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and to show how thesame may be carried into effect, embodiments of the present inventionwill now be described by way of example only with reference to theaccompanying drawings, in which:

FIGS. 1(a) to 1(e) illustrate a method of fabricating a compositesemiconductor component according to an embodiment of the presentinvention;

FIGS. 2(a) to 2(e) illustrate a method of fabricating a compositesemiconductor component according to another embodiment of the presentinvention; and

FIGS. 3(a) to 3(d) illustrate a method of fabricating a compositesemiconductor component according to another embodiment of the presentinvention.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

FIGS. 1(a) to 1(e) illustrate a method of fabricating a compositesemiconductor component according to an embodiment of the presentinvention.

In step 1, a layer of polycrystalline CVD diamond material 2 is grown ona silicon substrate 3. On cooling after diamond growth the substratebows due to a thermal expansion coefficient mismatch between the siliconsubstrate and the diamond material as illustrated in FIG. 1(a).

In step 2 the silicon substrate 3 is removed, e.g. via etching. Duringprocessing to thin and remove the silicon substrate 3 the stress in thesilicon increases and as the layer of silicon reaches a thickness ofapproximately 100 μm the composite tends to flip to bow the other waysuch that the thin silicon layer is placed in tension as illustrated inFIG. 1(b).

In step 3, further processing can remove the remaining silicon leaving abowed wafer of CVD diamond material as illustrated in FIG. 1(c).

A key feature is that the wafer of polycrystalline CVD synthetic diamondmaterial 2 is bowed such that it comprises a convex face 4 and a concaveface 6. The magnitude of bowing B may be measured as the height of themid-point of the substrate relative to a plane defined by the edge ofthe substrate. Alternatively, the magnitude of bowing B may be measuredas the radius of curvature of the substrate.

The magnitude of bowing will depend on the thickness of the originalsilicon substrate and the thickness of the polycrystalline CVD diamondlayer grown thereon. Furthermore, using different substrate compositionswill vary the thermal expansion coefficient mismatch and rigidity of thesystem. As such, the diamond growth process can be tailored to generatea range of bowing magnitudes. Generally, the convex bow of the substrateis larger for thinner wafers of polycrystalline CVD diamond.

In step 4, a layer of compound semiconductor material 8 is grown on theconvex face 4 of the bowed polycrystalline CVD diamond substrate 2 via achemical vapour deposition technique at a growth temperature T(typically in excess of 1000° C.) to form a bowed compositesemiconductor component 10 as illustrated in FIG. 1(d). The bowedcomposite semiconductor component 10 comprises the layer of compoundsemiconductor material 8 on the convex face 4 of the bowedpolycrystalline CVD diamond substrate 2.

In step 5, the bowed composite semiconductor component 10 is cooled. Thecompound semiconductor material has a higher average thermal expansioncoefficient than the polycrystalline CVD synthetic diamond materialbetween the growth temperature T and room temperature providing athermal expansion mismatch ΔT_(ec). On cooling, the layer of compoundsemiconductor material contracts more than the wafer of polycrystallineCVD synthetic diamond material due to the thermal expansion mismatchΔT_(ec). Contraction of the layer of compound semiconductor materialpulls the polycrystalline CVD synthetic diamond material into a flatconfiguration 12 as illustrated in FIG. 1(e). Movement of thepolycrystalline CVD synthetic diamond material from a bowed state to aflat state allows the compound semiconductor material to contract thusavoiding a large tensile stress being introduced into the compoundsemiconductor layer during cooling. Furthermore, if a suitablecombination of materials and layer thicknesses is selected the layer ofcompound semiconductor material contracts on cooling by an amount whichoff-sets bowing in the substrate thus pulling the bowed compositesemiconductor component into a flat configuration with the layer ofcompound semiconductor material having a tensile stress after cooling ofless than 500 MPa, more preferably less than 450 MPa, 400 MPa, 350 MPa,300 MPa, 250 MPa, or 210 MPa at room temperature (25° C.). A flat,crack-free, low strain compound semiconductor layer on a substratecomprising a wafer of polycrystalline CVD synthetic diamond material isthus produced.

As indicated above, a suitable combination of materials and layerthicknesses must be selected such that the layer of compoundsemiconductor material contracts on cooling by an amount which off-setsbowing in the substrate to yield a flat, crack-free, low strain compoundsemiconductor layer on a substrate comprising a wafer of polycrystallineCVD synthetic diamond material. For example, if the diamond layer is toothick or the compound semiconductor layer is too thin then the thermallyinduced compressive stress generate in the diamond wafer by contractionof the compound semiconductor layer will be insufficient to pull thebowed substrate into a flat configuration and on cooling tensile stresswill build up in the overlying semiconductor layer. Similarly, if thesubstrate is too bowed, or alternatively insufficiently bowed, then aflat, crack-free, low strain compound semiconductor layer will not beachieved after cooling. The amount of bowing B required will bedependent on the thickness of the diamond and semiconductor layerst_(d), t_(sc), and the magnitude of the thermal expansion coefficientmismatch ΔT_(ec). As such, the parameters B, t_(d), t_(sc), and ΔT_(ec)are all interrelated with the magnitude of any one of the parametersbeing dependent on the magnitude of the other parameters. However, for agiven compound semiconductor material the thermal expansion coefficientmismatch ΔT_(ec) will be fixed. A range of bow magnitudes and layerthicknesses may then be tested to achieve the desired result.

In the embodiment illustrated in FIG. 1 the substrate for compoundsemiconductor growth is formed of a wafer of polycrystalline diamondmaterial with the compound semiconductor material deposited directlythereon. However, in an alternative arrangement the bowed substratecomprises a layer of single crystal material on the convex face of thebowed substrate, the layer of compound semiconductor material beinggrown on said layer of single crystal material. Advantageously, thelayer of compound semiconductor material is disposed close to thediamond material in order to achieve efficient heat spreading, e.g. aspacing between the compound semiconductor material and the diamondmaterial being no more than 5 μm, 3 μm, 2 μm, or 1 μm. As such, one ormore intermediate bonding layers disposed between the compoundsemiconductor and diamond material should be made very thin, i.e., nomore than 5 μm, 3 μm, 2 μm, or 1 μm in total thickness. The one or moreintermediate bonding layers may be formed from single crystal materialssuch as silicon, silicon carbide, or nitrides suitable for epitaxialgrowth of compound semiconductors.

FIGS. 2(a) to 2(e) illustrate a method of fabricating a compositesemiconductor component according to another embodiment of the presentinvention in which a thin layer of silicon is provided between thecompound semiconductor layer and the wafer of polycrystalline CVDdiamond material.

In step 1 a diamond wafer 20 is grown on a silicon-on-insulator (SOI)substrate 22. The SOI substrate 22 comprises a thin layer of silicon 24,a buried SiO₂ layer 26, and a thicker supporting layer of silicon 28. Oncooling after diamond growth the substrate bows due to a thermalexpansion coefficient mismatch between the silicon substrate and thediamond material as illustrated in FIG. 2(a).

In step 2 the supporting layer of silicon 28 is thinned, e.g. viaetching. During processing to thin the silicon wafer the stress in thesilicon increases and as the layer of silicon reaches a thickness ofapproximately 100 μm the composite tends to flip to bow the other waysuch that the thin silicon layer 24 is placed in tension as illustratedin FIG. 2(b). In step 3, further processing can remove the remainingsilicon and the SiO₂ layer thus leaving a thin silicon layer 24 adheredto the CVD diamond material 20 without any significant cracking asillustrated in FIG. 2(c).

In step 4, the bowed diamond-silicon substrate can then be used as asubstrate for compound semiconductor growth in a similar manner to thatillustrated in FIG. 1. FIG. 2(d) illustrates the growth of a compoundsemiconductor layer 30 on the thin silicon layer 24. In step 5, thecomposite diamond-silicon-compound semiconductor component is cooling.As previously described, the thermal expansion coefficient mismatchbetween the compound semiconductor layer 30 and the diamond wafer 20causes the structure to be pulled into a flat configuration if asuitable combination of layer thicknesses and starting strain in thediamond wafer is selected as illustrated in FIG. 2(e).

As described in the background section, one problem with fabricating athin silicon layer on a diamond carrier wafer is that as the silicon isthinned down as illustrated in FIGS. 2(b) and 2(c), cracking can occurin the thin layer such that although a thin layer is achieved it is notof high quality. The present inventors believe that the aforementionedproblem of cracking of the silicon material (or other suitable singlecrystal layers such as carbides and nitrides) as the material is thinnedto less than 100 μm is a result of thermally induced stress generated bythe CVD diamond growth process and caused by a mismatch in thermalexpansion coefficient between the polycrystalline diamond material andthe single crystal wafer material. For example, silicon has a muchhigher thermal expansion coefficient than diamond (at least attemperatures below about 700° C.) such that on cooling after CVD diamondgrowth the silicon wafer contracts more than the polycrystalline diamondcausing bowing as illustrated in FIG. 2(a). During processing to thinthe silicon wafer the stress in the silicon increases and as the layerof silicon reaches a thickness in a range from half the thickness of thediamond wafer to twice the thickness of the diamond wafer, e.g.approximately 100 μm, the composite tends to flip to bow the other waysuch that the thin silicon layer is placed in tension as illustrated inFIG. 2(b). This tension tends to cause the thin silicon layer to crack.

The aforementioned problem can be partially addressed by controlling thetemperature of the growth surface of the polycrystalline CVD diamondlayer during growth such that a temperature difference at a growthsurface between an edge and a centre point thereof is maintained to beno more than 80° C., 60° C., 40° C., 20° C., 10° C., 5° C., or 1° C.Such temperature control can aid in alleviate problems of thermallyinduced stresses leading to cracking.

The present inventors have also found that the mechanical stiffness ofthe substrate wafer must be sufficiently large to alleviate the problemof bowing and cracking of the silicon material during processing of thesubstrate wafer after CVD diamond growth thereon. The present inventorshave found that the required level of mechanical stiffness can beachieved by providing a substrate wafer which is relatively thick(compared with its lateral width). Otherwise, even if the temperatureacross the growth surface is controlled to vary by no more than 80° C.,plastic deformation of the substrate wafer still occurs. For example,the aspect ratio of the substrate wafer, defined by a ratio of thicknessto width, should be no less than 0.25/100, 0.30/100, 0.40/100, 0.50/100,0.60/100, 0.70/100, 0.80/100, 0.90/100, or 1.0/100. However, if thesubstrate wafer becomes too thick then this adds significant expense. Assuch, in practice the substrate wafer has an aspect ratio no more than10/100, 8/100, 6/100, 4/100, or 2/100.

Despite the above modifications, cracking of the thin silicon layer ofmaterial during thinning of the substrate wafer can still beproblematic. The present inventors have found that the thin siliconlayer can be made more resistant to cracking under the tensioningmechanism illustrated in FIG. 2(b) if a relatively thick layer of SiO₂is introduced into the silicon wafer prior to CVD diamond growththereon. While not being bound by theory it is postulated that thisreduction in cracking may be due to the relatively thick SiO₂ layerfunctioning as a crack stop and/or the relatively thick SiO₂ layerfunctioning to pre-compress the thin overlying layer of single crystalsilicon material during CVD diamond growth due to its lower thermalexpansion coefficient such that it is more resistant to tensile stressinduced during processing of the silicon wafer to achieve a thin layer.In either case, it is important that the SiO₂ layer should have athickness sufficient to fulfil either of these functions.

As shown in FIG. 2(a) bowing of the substrate after CVD diamond growththereon can still occur as described previously. Furthermore, asdescribed previously, during processing to thin the silicon wafer thestress in the silicon increases and as the layer of silicon reaches athickness in a range from half the thickness of the diamond wafer totwice the thickness of the diamond wafer, e.g. approximately 100 μm, thecomposite tends to flip to bow the other way such that the thin siliconlayer is placed in tension as illustrated in FIG. 2(b). However, even ifcracks form in the silicon handle wafer these cracks are prevented frompropagating into the thin silicon layer adjacent the CVD diamondmaterial by the relatively thick SiO₂ layer. Further processing canremove the remaining silicon and the SiO₂ layer thus leaving a thinsilicon layer adhered to the CVD diamond material without anysignificant cracking. As illustrated in FIG. 2(c).

FIGS. 3(a) to 3(d) illustrate a method of fabricating a compositesemiconductor component according to yet another embodiment of thepresent invention.

In step 1, a flat polycrystalline CVD diamond wafer 30 is provided asillustrated in FIG. 3(a). Such a polycrystalline CVD diamond wafer 30may be fabricated by CVD diamond growth on a refractory metal substrate.

In step 2, a material 32 having a thermal expansion coefficient lowerthan that of the polycrystalline CVD diamond wafer (such as SiO₂) isadhered to one side of the polycrystalline CVD diamond wafer asillustrated in FIG. 3(b).

In step 3, a compound semiconductor layer 34 is grown on an oppositeside of the polycrystalline CVD diamond wafer 30 to the layer of SiO₂32. As the composite diamond-SiO₂ substrate is raised to the growthtemperature of the compound semiconductor layer the compositediamond-SiO₂ substrate bows due to the thermal expansion coefficientmismatch between the diamond and SiO₂. As the SiO₂ has a lower thermalexpansion coefficient than diamond then the diamond 30 forms a convexgrowth surface on which the compound semiconductor 34 is grown asillustrated in FIG. 3(c).

In step 4, on cooling, the compound semiconductor layer 34 contractsmore than the diamond 30 and the diamond 30 contracts more than the SiO₂layer 32 thus pulling the composite structure back to a flatconfiguration in a similar manner to previous embodiments and asillustrated in FIG. 3(d).

FIGS. 1 to 3 have illustrated three different ways of implementingembodiments of the present invention. However, other alternatives mayalso be envisaged. For example, the polycrystalline CVD diamond wafermay be mechanically bowed prior to growth of a compound semiconductorthereon. Alternatively, the low thermal expansion coefficient material32 shown in FIG. 3 may be combined with the approaches illustrated inFIG. 1 or 2 to control bowing of the substrate prior to compoundsemiconductor growth and straightening on cooling after compoundsemiconductor growth. Alternatively still, a plurality of layers havingdifferent thermal expansion coefficients may be adhered to thepolycrystalline CVD diamond wafer to control bowing and straightening.The key feature of all these embodiments is that on cooling aftercompound semiconductor growth the structure allows the compoundsemiconductor layer to contract without building up excessive tensilestress.

Advantageously, embodiments of the present invention will utilizediamond material may have a thermal conductivity equal to or greaterthan 600 Wm⁻¹K⁻¹, 800 Wm⁻¹K⁻¹, 1000 Wm⁻¹K⁻¹, 1200 Wm⁻¹K⁻¹, or 1400Wm⁻¹K⁻¹. Such high quality diamond material can be fabricated using amicrowave plasma method. As such, embodiments of the present inventionprovide high quality diamond material combined with high quality, lowstrain compound semiconductor material in close proximity resulting in abetter performance semiconductor device.

Suitable wafers of polycrystalline CVD diamond material can have adiameter in a range: 20 mm to 300 mm; 20 mm to 250 mm; 20 mm to 200 mm;20 mm to 160 mm; 40 mm to 140 mm; 60 mm to 120 mm; 80 mm to 120 mm; or90 mm to 110 mm. As previously indicated, a more pronounced bowing canbe achieved by fabricating thin wafers of polycrystalline CVD diamond.As such, the wafers of polycrystalline CVD diamond material may be grownto a thickness in the range: 25 μm to 450 μm; 25 μm to 400 μm; 25 μm to350 μm; 25 μm to 300 μm; 25 μm to 250 μm; 25 μm to 200 μm; 25 μm to 150μm; 40 μm to 130 μm; or 50 μm to 100 μm. The wafer of synthetic diamondmaterial may be formed of a free-standing synthetic diamond wafer or awafer which comprises a layer of synthetic diamond material on a supportsubstrate.

While previous embodiments have been described in relation topolycrystalline CVD diamond wafers, it is also envisaged that themethodology of the present invention can also be applied equally well tosingle crystal diamond wafers, e.g. single crystal CVD diamond wafers.It has recently been demonstrated that compound semiconductors such asGaN can be grown directly on both polycrystalline CVD diamond wafers andsingle crystal diamond wafers. The same thermally induced tensile stressproblem will equally exists for growth of compound semiconductormaterial on substrates comprising single crystal diamond material. Assuch, bowing a single crystal diamond wafer as previously described willalleviate this problem in an analogous manner to that described forpolycrystalline CVD diamond wafers.

The layer of single crystal compound semiconductor may have a chargemobility no less than 1000 cm²V⁻¹s⁻¹, 1200 cm²V⁻¹s⁻¹, 1400 cm²V⁻¹s⁻¹,1600 cm²V⁻¹s⁻¹, 1800 cm²V⁻¹s⁻¹, or 2000 cm²V⁻¹s⁻¹. While such chargemobilities of compound semiconductor layers have previously beenachieved on non-diamond substrates and may be achieved on diamondsubstrates when the semiconductor layer is not in good thermal contactwith the diamond substrate layer, the combination of a relatively thick,high thermal conductivity diamond layer in combination with a highquality, low strain compound semiconductor layer, with both layersdisposed in very close proximity to provide good thermal contact hasproved difficult to date for the reasons explained in thisspecification.

EXAMPLES

Three different substrates have been fabricated, each comprising apolycrystalline CVD diamond wafer with a thin layer of single crystal{111} oriented silicon disposed thereon. The first substrate wasfabricated to have a highly convex silicon growth surface, the secondsubstrate was fabricated to have a slightly convex (nearly flatconfiguration), whereas the third substrate was fabricated to have aslightly concave silicon growth surface. All three substrates wereapproximately 50 to 60 μm thick, the silicon layer of each substratebeing approximately 2 μm thick. The thin silicon layer of each substratewas formed by etching back a thicker substrate in a similar manner tothat illustrated with reference to FIG. 2. The etching provided ahydrogen terminated silicon growth surface which is ideal for MOCVDgrowth of semiconductor material thereon.

Compound semiconductor layers were grown on the silicon growth surfaceof each of the three diamond-silicon substrates. The compoundsemiconductor layers included a stacked layer structure comprising AN,AlGaN, and GaN layers.

After fabrication, the composite structures were analysed using a knownmicro-Raman technique to measure the tensile stress in the GaN layer ofeach of the three components. The slightly concave substrate and thenearly flat substrate resulted in a relatively high tensile stress inthe GaN layer while the highly convex substrate resulted in a lowtensile stress in the GaN layer of below 210 MPa.

While this invention has been particularly shown and described withreference to preferred embodiments, it will be understood to thoseskilled in the art that various changes in form and detail may be madewithout departing from the scope of the invention as defined by theappendant claims.

The work leading to this invention has received funding from the[European Community's] [European Atomic Energy Community's] SeventhFramework Programme ([FP7/2007-2013] [FP7/2007-2011]) under grantagreement n° [214610].

The invention claimed is:
 1. A method of fabricating a compositesemiconductor component, the method comprising: (i) disposing asynthetic diamond layer on a wafer and cooling the synthetic diamondlayer so that the synthetic diamond layer and wafer form a first bowedsubstrate; (ii) removing at least a portion of the wafer of the firstbowed substrate so that the first bowed substrate flips to bow in anopposite direction to thereby form a second bowed substrate, the secondbowed substrate has a convex face and a concave face; (iii) growing alayer of compound semiconductor material on the convex face of thesecond bowed substrate at a growth temperature to form a bowed compositesemiconductor component, the compound semiconductor material having ahigher average thermal expansion coefficient than the synthetic diamondmaterial between the growth temperature and room temperature; and (iv)cooling the bowed composite semiconductor component, wherein the layerof compound semiconductor material contracts more than the syntheticdiamond layer during cooling due to a thermal expansion mismatch betweenthe compound semiconductor material and a material of the syntheticdiamond layer and wherein the layer of compound semiconductor materialcontracts on cooling by an amount which off-sets bowing in the secondbowed substrate thus pulling the bowed composite semiconductor componentinto a flat configuration.
 2. A method according to claim 1, wherein,after step (iv), the layer of compound semiconductor material has atensile stress of less than 500 MPa.
 3. A method according to claim 1,wherein a thickness of the synthetic diamond layer is in a range of 25μm to 450 μm.
 4. A method according to claim 1, wherein the waferincludes a silicon layer and the step of removing at least a portion ofthe wafer includes removing the silicon layer.
 5. A method according toclaim 1, wherein the wafer further includes a first silicon layer, asilicon dioxide layer and a second silicon layer and the step ofremoving the at least a portion of the wafer includes removing the firstsilicon layer.